This invention relates to a drum brake of the inner shoe type which is particularly adapted for use in automotive vehicles and which has a mechanical self-adjusting device for reducing an excessive clearance developing between the brake lining and the drum wall. The self-adjusting device extends between the two brake shoes and is operatively connected therewith. The self-adjusting device has a push rod of constant length, one end of which is in engagement with a first brake shoe or with a brake lever which forms part of a parking brake and which is pivotally connected to the first brake shoe. The other end of the push rod is in engagement with the second brake shoe with the intermediary of an adjusting wedge.
Drum brakes of the inner shoe type are in general so dimensioned that in the released state of the brake, the brake linings do not engage the drum wall. The distance between the drum wall and the work face of the lining should be as small as possible to ensure that the brake fluid quantities required for brake actuation are small and the required brake pedal travel remains short. The necessary clearance between the brake lining and the drum wall to ensure a freedom from interference by the brake in the released state is, in drum brakes, measured across the center of the brake parallel to the wheel cylinder and is designated hereafter as "air gap". This air gap continuously increases during the wear of the brake lining as it engages the brake drum during the braking operation and further, the air gap is temporarily increased because of heat expansion of the brake drum. These circumstances cause an increase of the brake pedal travel.
In view of the above, care has to be taken that the brake shoes are adjusted sufficiently early so that a permissible predetermined maximum air gap is not exceeded. In case of a tardy adjustment of the brake, the risks are high that the then still available brake pedal travel is no longer sufficient to effect the required pressing of the brake shoes against the brake drum. Consequently, the required braking effect in crucial moments may not be present. For this reason it is conventional to provide within the drum brake of the inner shoe type, a device which automatically adjusts the brake shoes, so that between the brake lining and the drum wall there will be present at all times an as small an air gap as possible despite the progressive wear of the brake lining.
Mechanical self-adjusting devices of the above-outlined type are known in a wide variety. In addition to the best known devices of this type which have a locking pawl (such as disclosed in British Pat. No. 1,191,320) or a tooth lock (such as disclosed in German Pat. No. 861,211) wherein the effective length of a two-part push rod arranged between the two brake shoes is increased by rotating an adjusting nut by means of a pawl lever engaging into the teeth of the adjusting nut or by means of pulling outwardly a toothed bolt from a toothed sleeve, there are further known adjusting devices (such as disclosed, for example, in German Pat. No. 1,029,243 and German Laid-Open Application (Offenlegungsschrift) No. 2,010,907) in which a push rod of constant length engages, with one end, one of the brake shoes or components thereof and with the other end engages the other brake shoe with the intermediary of an adjusting wedge. The adjustment of the brake shoes is effected in the last-named adjusting devices by causing the wedge to penetrate deeper into the wedge-shaped space between the push rod, on the one hand, and the adjoining brake shoe, on the other hand, as the lining wear increases. The wedge is urged into the wedge-shaped space by a spring device. In these known arrangements one flank of the adjusting wedge is provided with ratchet teeth which cooperate with corresponding ratchet teeth provided on the push rod or the brake shoe. As long as, during the actuation of the brake operating device (wheel cylinder) the relative motion of the brake shoes is smaller than the depth of the ratchet teeth, the adjusting wedge maintains its position unchanged. The toothed adjusting wedge is further moved by the spring device into the wedge-shaped space between the push rod and the brake shoe only when the brake lining is worn to such an extent that upon operating the brake, the above-noted relative motion is greater than the depth of the ratchet teeth (which means that the air gap has increased correspondingly). This advance of the adjusting wedge is of such extent that it corresponds to the width of one or several ratchet teeth. The number of ratchet teeth is dependent upon the number of tooth heights by which the brake shoes have moved away from one another. Similarly to the earlier-mentioned adjusting devices which have a pawl lock or a tooth lock, in the arrangement operating with an adjusting wedge, the brake shoe adjustment is always effected stepwise. This means that similarly, the air gap too, can only be reduced in a stepwise manner. In drum brakes of the inner shoe type which have a brake lever which forms part of a parking brake (hand brake) and which is articulated to one of the brake shoes, the push rod engages the above-noted adjusting device and the brake lever and transmits the operating force, introduced by the brake lever, to the other brake shoe. Upon actuation of the brake lever, generally very high forces are generated which are also effective in the teeth of the adjusting device. Therefore, there is always present some risk that the teeth will be damaged, thus endangering a properly operating self-adjustment of the brake shoes.
There is further known a drum brake (such as disclosed in U.S. Pat. No. 2,322,061), wherein a toothless adjusting wedge is utilized for the mechanical self-adjustment of the air gap. This known wedge-shaped adjusting device has a relatively complicated structure. It has two wedge-shaped components, one of which is situated between the brake shoe and the push rod and varies the effective length of the push rod. In order to displace this wedge-shaped component in the required manner during brake lining wear, there is provided a second wedge-shaped component cooperating with further components. The first wedge-shaped component is pressed against the push rod and the adjacent brake shoe by means of a return spring attached to both brake shoes. This pressing (tensioning) is relaxed (released) upon each braking operation and the first wedge-shaped component is adjusted at times. Thus, as the lining wear increases, the return force of the return spring varies accordingly. The desired air gap in this known drum brake is initially set during installation by means of a special cam plate.